flbe  Ulntversitg  of  Chicago 


HVIt< 


PHYTOGEOGRAPHY  OF  THE  EASTERN 
MOUNTAIN  FRONT  IN 
COLORADO 


A DISSERTATION 
SUBMITTED  TO  THE  FACULTY 
OF  THE  OGDEN  GRADUATE  SCHOOL  OF  SCIENCE 
IN  CANDIDACY  FOR  THE  DEGREE  OF 
DOCTOR  OF  PHILOSOPHY 

DEPARTMENT  OF  BOTANY 


BY 

ARTHUR  GIBSON  VESTAL 


Private  Edition,  Distributed  By 
THE  UNIVERSITY  OF  CHICAGO  LIBRARIES 
CHICAGO,  ILLINOIS 


Reprinted  from 

The  Botanical  Gazette,  Vol.  LXVIII,  No.  3 
September  1919 


Uhc  Ulnlverstts  of  Chicago 


PHYTOGEOGRAPHY  OF  THE  EASTERN 
MOUNTAIN  FRONT  IN 
COLORADO 


A DISSERTATION 
SUBMITTED  TO  THE  FACULTY 
OF  THE  OGDEN  GRADUATE  SCHOOL  OF  SCIENCE 
IN  CANDIDACY  FOR  THE  DEGREE  OF 
DOCTOR  OF  PHILOSOPHY 

DEPARTMENT  OF  BOTANY 


BY 

ARTHUR  GIBSON  VESTAL 


Private  Edition,  Distributed  By 
THE  UNIVERSITY  OF  CHICAGO  LIBRARIES 
CHICAGO,  ILLINOIS 


Reprinted  from 

The  Botanical  Gazette,  Vol.  LXVIII,  No.  3 
September  1919 


Digitized  by  the  Internet  Archive 
in  2017  with  funding  from 

University  of  Illinois  Urbana-Champaign  Alternates 


https://archive.org/details/phytogeographyofOOvest 


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5ZJ.9  7St 

V (e3  j=> 


OLUME  LXVIII 


NUMBER^ 


THE 

Botanical  Gazette 

SEPTEMBER  igig 

PHYTOGEOGRAPHY  OF  THE  EASTERN  MOUNTAIN- 
FRONT  IN  COLORADO 

' I.  PHYSICAL  GEOGRAPHY  AND  DISTRIBUTION 
OF  VEGETATION 

CONTRIBUTIONS  FROM  THE  HULL  BOTANICAL  LABORATORY  251 
Arthur  G.  Vestal 
(with  seventeen  figures) 

Introduction 

The  plant  geography  of  a region  is  the  effect  of  the  working  of 
present  and  former  environmental  influences  upon  the  floras  and 
vegetation-complexes  which  exist  and  have  existed  within  the  region 
and  in  the  regions  adjoining.  The  region  of  present  study,  lying 
as  it  does  in  the  transition  belt  between  two  great  geographic 
divisions  of  North  America,  the  Great  Plains,  or  western  part  of  the 
prairie  region,  and  the  Rocky  Mountains,  has  some  of  the  char- 
acters of  both;  others  of  its  physical  and  vegetational  features  are 
transitional,  intermediate;  and  it  has  certain  peculiarities,  differ- 
ing thus  from  the  regions  on  either  side.  Since  climatic  variation, 
differences  of  soil  and  of  topography,  and  multiformity  of  vegetation- 
types  are  considerable,  the  plant-covering  of  the  area  is  a complex 
of  many  diverse  types.  Descriptive  accounts  of  the  plant  associa- 
tions of  plains  and  foothills  have  already  been  published  (17,  18), 
so  that  the  present  article  may  deal  more  particularly  with  geo- 
graphic description  and  geographic  relations. 

153 


Ia 


154 


BOTANICAL  GAZETTE 


[SEPTEMBER 


Fig.  i. — Map  of  southern  Rocky 
Mountains,  except  westernmost  ranges; 
mountain  areas  shaded;  names  of  areas 
indicated  by  numbers  are:  i,  Laramie 
Mountains;  2,  Medicine  Bow  Range;  3, 
low  mountain  area  connecting  Laramie 
and  Front  ranges;  4,  foothills  of  Poudre 
River  area;  5,  Front  Range;  6,  Rampart 
Range;  7,  Pike’s  Peak  highland;  8,  Park 
Range;  9,  Saguache  Range;  10,  Upper 
Arkansas  Valley  (between  9 and  8); 
11,  low  mountains;  12,  Wet  Mountain 
Valley;  13,  Sangre  de  Cristo  Range;  14, 
Wet  and  Greenhorn  mountains;  15, 
Huerfano  Park;  16,  southern  sedimentary 
plateau;  17,  Culebra  Range;  18,  Spanish 
Peaks  highland;  19,  Raton  mesas. 


While  in  general  the  plains 
and  mountains  contrast  rather 
sharply  at  their  junction,  this 
is  not  always  true;  the  moun- 
tain-front is  a transition  zone 
in  places  a number  of  miles 
broad  rather  than  a line.  It  is 
not  determined  alone  by  alti- 
tude, by  topography,  by  char- 
acter of  the  bedrock,  or  by 
climate;  it  is  the  resultant  of 
all  of  these.  For  the  sake  of 
clearness  the  foothills  may  be 
described  as  the  drier  and  less 
elevated  (about  5800-8000  ft.) 
part  of  the  mountain  plateau, 
with  vegetation  composed  of 
grassland,  scattered  rock  pines, 
and  a few  other  trees  ( foothill 
zone , Ramaley  8).  Except  in 
the  southern  “ sedimentary  pla- 
teau” (fig.  i),  perhaps  rather 
to  be  considered  part  of  the 
mountain-front  area,  the  foot- 
hills may  be  said  to  comprise  the 
granitic  hills  of  the  mountain- 
mass  proper;  while  to  the 
mountain-front  zone  may  be 
assigned  the  upturned  sedimen- 
tary hogbacks  and  longitudinal 
valleys,  sedimentary  outcrops, 
buttes  and  broken  plateaus,  and 
the  mesas  and  upper  parts  of 
the  debris-covered  slope  to  the 
plains.  The  vegetation  is  of  the 
greatest  variety.  The  plains 
proper  may  be  said  to  com- 
mence where  the  mixed  soil  and 


1919]  VESTAL— PHYTOGEOGRAPHY  OF  COLORADO  155 

vegetation  of  the  detrital  outwash  from  the  hills  is  succeeded  by 
the  fine  soil  and  mostly  short-grass  vegetation  of  the  shale  beds 
covering  most  of  the  Great  Plains  surface. 

Plan  of  presentation. — The  writer  has  been  much  influenced  by 
the  work  of  DAvis  (1)  on  the  geography  of  the  Colorado  Front 
Range,  a regional  presentation  and  particularly  relevant  in  this 
study,  since  the  area  considered  is  so  nearly  the  same.  Davis’ 
systematic  treatment  avoids  repeating  descriptions  of  frequently 
encountered  land-forms  by  recognizing  their  common  features  and 
giving  each  a brief  characterization  and  a name,  thus  identifying 
them  when  mentioned  later.  Minor  differences  of  detail  are  not 
considered  in  the  condensed  treatment  thereby  made  possible. 
In  a regional  study,  in  which  numerous  elements  form  an  intricate 
complex,  this  omission  of  detail  is  essential.  As  the  physical 
geographer  refers  land-forms  to  types  (mental  counterparts  of 
physical  realities),  so  in  a regional  study  of  plant  geography  one 
may  refer  forms  of  vegetation  to  types  which  are  the  same  over 
considerable  areas.  This  is  a common  practice  in  ecological 
classification,  but  many  studies  of  limited  areas  of  vegetation  have 
characterized  the  plant  communities  without  regard  to  geographic 
orientation.  If  possible,  local  representations  or  variants  of  wide- 
spread associations  should  be  recognized  as  such.  The  characteri- 
zation of  the  relatively  few  widespread  and  important  vegetation- 
types  makes  it  possible  to  systematize  plant  geography.  This 
systematic  treatment  emphasizes  the  common  features,  the  resem- 
blances of  similar  plant  communities,  but  the  differences,  when 
worthy  of  note,  can  always  be  stated  in  addition.  The  section  of 
this  study  which  is  here  published  is  the  systematic  part,  which 
establishes  the  types  of  topography,  soil,  climate,  and  vegetation  as 
developed  in  the  region  or  in  parts  of  it.  It  will  be  followed  by  a 
regional  section,  which  describes  the  physical  and  vegetational 
features  “in  their  actual  spatial  relations,”  to  use  the  words  of 
Davis,  and  by  parts  dealing  with  general  geographic  and  develop- 
mental relations  of  the  vegetation. 

Physical  features 

The  area  studied  is  the  eastern  front  of  the  Rocky  Mountains 
in  Colorado,  of  which  the  most  characteristic  part  is  the  Front 


BOTANICAL  GAZETTE 


[SEPTEMBER 


156 

Range.  This  has  been  studied  by  many  geographers,  more  recently 
by  Davis.  The  Front  Range  has  been  described  by  him  as  a sub- 
maturely  dissected  upland  of  crystalline  rocks,  elevated  above  the 
plains  to  the  east  by  a long  north-south  monoclinal  fold.  The 
tops  of  most  of  the  hills  form  the  remains  of  a pfcneplaned  surface, 
the  result  of  the  erosion  following  the  uplift,  with  complete  removal 
of  the  sedimentary  layers  from  the  raised  area  on  the  west.  A few 


Fig.  2. — Davis’  block  diagram  of  Front  Range  (reproduced  with  author’s  per- 
mission from  1):  at  right  is  condition  following  first  uplift  with  monoclinal  fold; 
next  part  shows  peneplaned  upland  with  monadnocks  and  cuestas  (hogbacks);  third 
shows  entire  region  after  second  uplift;  last  block  on  left  shows  present  condition, 
with  glacier-carved  range-crest,  gently  sloping,  dissected,  crystalline  upland,  of  which 
lower  and  eastern  part  forms  foothills,  and  mountain-front,  with  sloping  crags,  cuestas, 
and  longitudinal  valleys;  outside  may  be  seen  debris-covered  terraces  and  broad 
valleys  of  streams  running  out  into  plains. 

monadnocks  surmount  the  general  level.  The  present  eastward 
inclination  of  the  old  peneplain  and  its  dissected  character  in  the 
crystalline  area,  and  the  removal  of  sedimentary  strata  of  the 
plains  to  a depth  far  below  the  foothills,  are  the  effects  of  a second 
uplift,  an  uparching  of  the  whole  region,  and  of  the  subsequent 
cycle  of  erosion.  Near  the  base  of  the  original  fold  the  sedi- 
mentary strata  are  sharply  upturned  against  the  outer  granitic 
slopes,  the  ends  of  the  resistant  strata  forming  ridges  and  sloping 
crags  (fig.  2). 


1919] 


VESTAL— PHYTOGEOGRAPHY  OF  COLORADO 


157 


The  outer  slope  to  the  plains  has  been  described  by  Johnson  (5) 
as  a debris-apron  or  composite  of  alluvial  fans,  of  which  the  profile 
is  that  of  a stream-grade,  rapidly  flattening  into  the  very  slight 
and  uniform  incline  of  the  Great  Plains.  The  graded  surface  is 
covered  by  unassorted  rock-waste  from  the  hills,  which  thins  out 
and  becomes  finer  in  texture  toward  the  east;  it  is  absent  from  most 
of  the  surface  of  the  plains,  which  is  of  fine  grained  residual  soil. 
This  grade  is  that  of  the  High  Plains;1  the  streams  have  very  gener- 
ally cut  below  it,  especially  near  the  mountains.  The  Platte  and 
Arkansas  rivers,  the  trunk-streams,  have  cut  very  broad  valleys 
in  the  soft  shales  of  the  plains.  The  north-south  valleys  of  their 
tributaries  which  parallel  the  mountain-front  are'  bordered  on  the 
east  by  escarpments  of  considerable  height  and  are  notable  geo- 
graphic features.  This  recent  downcutting,  where  working  in  soft 
shales  just  outside  the  foothills,  leaves  many  terraces,  remnants 
of  the  older  and  higher  stream-grade  levels ; their  covering  of  rock- 
waste  preserves  their  flat  tops.  They  are  generally  known  as 
“mesas”;  although  not  true  mesas,  the  term  is  convenient.2 
Where  the  upper  sedimentary  beds  consist  of  sandstone  or  limestone, 
extensive  plateau  areas  with  deep  canyons,  buttes  which  may  be 
numerous  or  scattered,  or  simple  escarpments  may  be  encountered. 
In  a few  places  igneous  intrusions  are  seen  as  dikes  or  as  basaltic 
layers  capping  larg£  mesas  (true  mesas  in  this  case).  From  these 
features  the  mountain-front  zone  derives  its  varied  character;  the 
mountain  upland  on  the  west,  and  the  plains  extending  far  to  the 
east,  are  of  less  irregular  structure. 

Arrangement  of  the  component  ranges  and  smaller  ridges  en 
echelon  is  a notable  feature  of  the  easternmost  line  of  mountains. 
Ranges  which  are  in  general  north  and  south  of  each  other  are 
themselves  oriented  with  the  northern  end  a little  to  the  west. 
Marvine  writes  (7,  p.  132): 

1 The  distribution  of  the  remnants  of  the  High  Plains  may  be  seen  in  a map  by 
Johnson  in  the  article  mentioned. 

2 A true  mesa  is  a tableland  capped  by  a more  resistant  stratum  which  keeps 
the  top  flat  by  retarding  erosion  except  on  the  sides.  The  debris-covered  terraces 
flanking  the  mountains  are  like  a true  mesa  in  that  the  rock-waste  layer  acts  as  a more 
resistant  cap. 


BOTANICAL  GAZETTE 


[SEPTEMBER 


158 


In  traveling  from  the  north  along  the  zone  of  hogbacks  lying  at  the  base  of 
the  mountains  southward,  the  traveler  finds  the  mountain-slope  directly  west 
of  him  falling  lower  and  lower  until  it  becomes  an  insignificant  ridge,  and 
finally  dies  away  in  the  plains. 

Passing  around  the  southern  end  of  the  diminishing  ridge  the  main 
mountain-slope  is  found  lying  several  miles  to  the  west,  and  separated  from  the 

ridge  by  a baylike  valley  extending  northward  behind  it The  ridges 

are  uplifted  or  anticlinal  folds,  the  valleys  depressed  or  synclinal  folds,  both 
dying  away  southward  into  the  flatness  of  the  plains. 

The  minor  embayments  due  to  echelon  arrangement  may  be 
made  out  only  in  a large  scale  map,  but  the  major  embayments  at 
the  south  end  of  the  Rampart  Range,  the  Pike’s  Peak  highland,  and 
the  Greenhorn  Mountains  can  easily  be  seen  in  fig.  1. 

A more  detailed  view  of  the  typical  land-forms  and  vegetation- 
forms  encountered  in  passing  from  mountains  to  plains  traverses 
the  several  north-south  zones  in  the  following  order:  first  the 
granitic  foothills;  then  the  transition  zone  of  the  mountain-front, 
with  its  upturned  ridges,  its  mesas  and  graded  slopes,  and  in  places 
its  plateau  areas,  buttes,  and  escarpments;  and  lastly  the  plains 
themselves. 

GRANITIC  FOOTHILLS 

The  mountain  plateau  is  in  most  places  submaturely  dissected, 
the  original  upland  level  being  represented  only  by  the  rounded  tops 
of  the  hills  (fig.  3).  Slopes  and  summits  are*  thinly  covered  with 
rock-waste.  Occasional  resistant  dikes  and  ledges  give  craggy 
exposures  of  massive  rock,  not  covered  by  any  soil  or  debris.  Below 
these,  or  on  the  sides  of  steeper  ravines,  are  talus  slopes  of  variously 
sized  rocks,  or  slides  of  “ granite-gravel.”3'  Table  I is  a synopsis 
of  topographic  areas  of  the  foothills  arranged  as  habitats,  and, 
correlated  with  these,  the  characteristic  vegetation-types.  Edaphic 
conditions  largely  determined  by  topography  (local  position  in 
relation  to  surroundings,  direction,  amount  of  slope,  and  soil  tex- 
ture) have  been  discussed  in  the  account  of  foothills  vegetation  (18). 

This  two-column  form  of  presentation  is  adopted  as  being 
concise,  as  emphasizing  relations  between  physiographic  and  onto- 
graphic  features  (the  environment  and  the  environed),  and  as  per- 
mitting a more  comprehensive  view  of  the  whole  complex  and  its 

3 Decomposed  granite  in  small  angular  fragments. 


1919]  VESTAL— PHYTOGEOGRAPHY  OF  COLORADO  159 

parts  than  can  be  obtained  by  the  linear  arrangement.  Geog- 
raphers will  note  that  topographic  areas  rather  than  land-forms 
are  used  as  the  units  of  area  of  physical  conditions  (habitats), 
since  land-forms,  such  as  mesas  and  ravines,  may  include  several 
topographic  areas  presenting  quite  diverse  environmental  con- 
ditions. Moreover,  a single  topographic  area,  even  if  physically 
uniform,  may  allow  the  growth  within  it  of  several  more  or  less 
distinct  vegetation-types. 


Fig.  3. — Maturely  dissected  foothills  near  Boulder  Creek:  pine-sprinkled,  rather 
than  forested,  surface  mostly  covered  with  dry  grassland. 


A brief  statement  concerning  mountain  parks  may  be  made. 
These  are  small  plains  or  flat  valleys  shut  in  on  all  sides  by  hills. 
They  are  not  well  developed  in  the  foothills  as  compared  with  the 
montane  zone.  They  are  mostly  formed  where  one  of  the  principal 
eastward  flowing  streams  is  joined  by  tributaries  from  valleys 
opening  into  the  park.  There  is  a single  outlet.  Many  of  the 
montane  parks  in  the  Front  Range  contain  the  terminal  moraines 
of  former  valley  glaciers  from  above,  and  their  topography  is  in 
large  measure  the  work  of  ice.  The  slight  gradient  causes  many 


160  BOTANICAL  GAZETTE  [September 

TABLE  I 

Topographic  areas  (habitats)  and  associated  vegetation-types 

IN  GRANITIC  FOOTHILLS  COMPLEX 


TOPOGRAPHIC  AREAS 

The  geographic  mean  is  that  pre- 
sented by  rather  exposed  and  xero- 
phytic  sloping  surfaces,  thinly  covered 
with  rock-waste  of  mixed  texture, 
rather  gravelly  and  with  surface 
rocks.  Local  departures  from  the 
general  condition  are  as  follows: 


1.  Exposed  rock  surfaces  (bowlders 
and  rock-walls) 

2.  Rock-crevices 

3.  Rock-strewn  detritus  slopes 


4.  Rock-talus 

5.  Compacted  granite-gravel  floors 
and  side-slopes 

6.  Loose  granite-gravel  floors, 
washes,  and  talus  (gravel-slides) 

7.  Mixed-soil  floors  and  detritus- 
slopes  (fine  soil  with  imbedded  and 
superficial  rock-fragments  of  various 
sizes) 

8.  Fine-soil  floors  and  detritus- 
slopes  (infrequent) 

9.  Less  xerophytic  side-slopes 
(mostly  north-facing,  mostly  of  con- 
siderable gradient,  and  best  developed 
in  valleys) 

10.  Narrow  mesophytic  ravines 
(best  developed  as  small  side-canyons, 
especially  on  the  south  side  of  east- 
ward flowing  main  streams) 


11.  Stream-sides  in  shaded  ravines 


12.  Stream-sides  in  open  canyon 
bottoms 


VEGETATION-TYPES 

The  general  ground-cover  is  mixed 
foothills  grassland  and  primitive 
grassland,  largely  of  grasses  and  herbs 
of  the  plains,  with  admixture  of  Rocky 
Mountain  herbs,  not  all  xerophytic. 
Scattered  rock  pines  and  plants  of 
the  mixed  shrub  association,  singly 
or  in  clumps,  dot  the  surface.  In 
special  habitats  occur: 

1.  Xerophytic  lichen  association 

2.  Selaginella,  shrubs  of  Jamesia 
and  Ribes,  rock  pine 

3.  Mixed  grassland,  and  mixed 
consocies  of  primitive  grassland,  with 
higher  proportion  of  woody  plants 
(rock  pines,  mixed  shrub,  Ceanothus , 
Arctostaphylos ) 

4.  Artemisia  frigida-Koeleria  con- 
socies of  primitive  grassland  (18) 

5.  Compacted  granite-gravel  con- 
socies of  primitive  grassland  (18)  with 
rosette  plants;  Arctostaphylos 

6.  Primitive  grassland,  with 
Geranium-Chrysopsis  consocies,  mat 
(rosette)  consocies  of  gravel-slides,  etc. 

7.  Foothills  mixed  grassland,  with 
addition  of  other  components,  Ceano- 
thus, sumac,  pine,  etc. 

8.  Foothills  mixed  grassland,  of  a 
form  approaching  plains  short-grass 

9.  Mixture  of  mixed  shrub,  rather 
less  xerophytic  mixed  grassland,  and 
pine  associations,  with  representatives 
of  canyon  forest  and  scattered  trees  of 
Pseudotsuga 

10.  Mesophytic  representations  of 
mixed  shrub,  Pseudotsuga , aspen, 
Symphoricarpos,  canyon  forest,  and 
mesophytic  grassland  associations. 
Mosses,  Saxifraga,  etc.,  in  wet  rock- 
crevices 

11.  Betula,  Alnus,  Corylus,  and 
Acer  glabrum  of  the  canyon  forest; 
shrubs;  moist-soil  herbs,  as  Herac- 
leum,  Rumex , etc. 

12.  Populus  angustifolia,  willows, 
etc. 


1919]  VESTAL— PHYTOGEOGRAPHY  OF  COLORADO  16 1 

meanders  and  oxbows  in  the  streams,  and  there  are  in  some  parks 
small  lakes  in  morainal  depressions.  The  stream-sides  are  fre- 
quently boggy,  with  meadows  adjoining.  The  parks  are  mostly 
treeless,  or  nearly  so,  and  show  no  signs  of  former  or  impending 
forestation.  The  exposed  dry  flats  are  covered  with  dry  grass- 
land, its  composition  depending  on  altitude  and  geographic  position 
chiefly.  Differences  in  soil  texture  cause  local  variation  of  the 
grassland,  but  this  is  less  marked  and  less  minutely  local  than  on 
the  hill  slopes.  Certain  lower  areas  are  occupied  by  meadow  and  ' 
sedge  communities,  and  the  rolling  surfaces  of  moraines  (in  montane 
parks)  are  variable  in  soil  texture,  soil  moisture,  and  in  the  compo- 
sition of  their  grassland  cover;  but  the  greater  part  of  park  floors 
is  well  drained,  flat,  and  quite  uniformly  covered  with  dry  grass- 
land. This  vegetation,  in  any  one  park,  forms  what  might  be 
called  a crystallization  of  the  grassland  of  the  neighboring  hills, 
whether  in  foothills  or  montane  zone,  in  view  of  the  comparative 
uniformity  of  the  grassland  of  the  flats  as  contrasted  with  that  of 
the  diversified  slopes  of  hill  topography.  The  lower  parks  have 
a grassland  cover  very  like  that  of  coarse  soil  in  the  mountain- 
front  area  or  in  the  plains  (see  description  of  Estes  Park  in  the 
regional  section) . The  higher  parks  have  fewer  plants  of  the  plains 
and  more  of  the  mountains.  There  is  a floristic  and  vegetational 
gradation  from  plains  grassland  through  the  lower  parks  to  mon- 
tane grassland  as  seen  in  the  higher  levels.  The  parks  thus  show 
a steplike  series  of  floristic  and  ecological  changes  with  altitude. 
Ramaley  (10,  n)  for  some  years  has  studied  park  vegetation, 
especially  in  Boulder  Park  at  Tolland,  Colorado,  on  South  Boulder 
Creek. 

TRANSITION  AREA  OR  MOUNTAIN-FRONT  ZONE 
The  sedimentary  rocks,  lying  upon  the  granite,  are  upturned  at 
the  monoclinal  fold,  and  are  seen  in  a horizontal  series  of  exposures 
of  strata,  the  lower  and  older  members  abutting  on  the  granitic 
foothills  to  the  west,  the  upper  formations  outcropping  in  order 
toward  the  east.  Since  the  tilting  at  the  mountain-front  is  for 
considerable  distances  greater  than  450  (locally  reaching  90°  and 
even  more,  resulting  in  overturns),  the  lower  formations  have 
narrower  zones  of  outcrop  than  the  upper  strata,  which  dip  so 
slightly  as  to  cover  areas  many  miles  wide  in  the  plains.  The 


162 


BOTANICAL  GAZETTE 


[SEPTEMBER 


narrow  zone  of  older  and  lower  strata  contains  alternating  resistant 
and  soft  members,  giving  rise  to  the  hogback  ridges  and  intervening 
valleys  already  mentioned,  while  the  newer  rocks  are  mostly  soft 
shales  and  sandstones,  giving  a flat  or  rolling  topography  over  the 
surface  of  the  plains,  with  occasional  escarpments  at  the  edges  of 
stream-valleys.  Both  angle  of  dip  and  hardness  of  rock,  therefore, 
contribute  to  a differentiation,  in  the  sedimentary  area  outside 
the  foothills,  of  a relatively  narrow  ridge-valley  mountain-front 
zone  from  the  very  broad  and  mostly  flat  plains  region. 

Just  outside  the  ridge-and- valley  zone  is  the  graded  slope  to  the 
plains,  covered  with  rock-debris  and  dissected  into  terraces  or 
mesas  of  varying  level.  In  places  along  the  mountain-front  the 
ridge-and- valley  topography  is  absent  or  poorly  developed,  either 
because  the  troughs  are  not  yet  carved  beneath  the  slope  from  the 
granitic  hills,  or  because  the  ridges  are  already  planed  (locally)  to  a 
graded  floor.  The  terraces  are  also  missing  from  certain  parts  of 
the  mountain-front.  The  topographic  complexes  of  the  ridge 
country  and  of  the  mesa  country  may  now  be  described  separately. 

The  hogback  ridges  (cuestas)  and  intervening  troughs 
(figs.  4,  5). — Two  of  the  numerous  sedimentary  strata  overlying 
the  crystalline  rocks  are  so  resistant  as  to  form  ridges  over  great 
lengths  of  the  mountain-front.  These  two  strata  are  of  such 
conspicuous  ^geographic  importance  that  they  merit  distinctive 
names  and  since  many  persons  know  them  by  their  geological 
names,  these  will  be  used  here  in  a geographic  capacity.  The 
Fountain  sandstone,  which  in  most  places  lies  directly  upon  the 
granites,  is  very  thick,  and  is  composed  of  dark  red,  rough  arkose 
materials,  variable  in  texture.  It  is  in  places  more  resistant  than 
the  granites,  so  that  side-gulches  tributary  to  the  east-flowing 
streams  of  the  foothills  are  common  in  the  granite  just  beneath 
the  Fountain.  Continuous  troughs  between  the  Fountain  and  the 
granite  are  not  frequent.  In  many  places  the  hard  red  sandstones 
form  broad  smooth-faced  crags  lying  upon  the  outer  foothill  slope, 
reaching  maximum  size  in  the  well  known  “flat-irons”  south  Qf 
Boulder  (fig.  6).  The  other  hard  stratum  is  the  massive  gray 
sandstone  known  as  the  Dakota.  It  is  separated  from  the  Fountain 
by  several  less  resistant  strata  of  considerable  aggregate  thickness, 


IQI91 


VESTAL— PHYTOGEOGRAPHY  OF  COLORADO 


163 


Figs.  4,  5. — Upturned  sedimentary  ridges  of  mountain-front  zone:  fig.  4,  eastward 
view  in  Perry  Park,  where  a broad  flat  valley  has  been  leveled  between  ridges  and 
outer  granitic  foothills;  floor  of  flat  is  of  compacted  angular  fragments;  vegetation 
is  primitive  grassland  alternating  with  scrub  oak;  Dawson  Butte  in  far  background; 
fig.  5,  southward  view,  between  Golden  and  Morrison,  of  longitudinal  valley  inside 
Dakota  hogback,  shown  on  left  in  long  curve. 


164 


BOTANICAL  GAZETTE 


[SEPTEMBER 


and  is  usually  seen  as  a bold  ridge  parallel  to  the  outer  slope  of 
the  foothills  some  distance  to  the  east.  The  term  “ hogback”  is 
familiarly  applied  to  the  steep  Dakota  cuesta. 

A deep  and  wide  trough  usually  extends  between  the  Fountain 
crags  and  the  Dakota  cuesta.  The  upper  part  of  the  east-facing 
slope  of  this  trough  is  the  outcrop  of  a “creamy  sandstone,”  which 
in  places  forms  prominent  outcrops,  or  even  strong  ridges,  as  at 
Morrison  at  the  mouth  of  Bear  Creek.  Just  east  of  and  below  the 
creamy  sandstone  is  an  easily  eroded  shale,  which  gives  its  rich 
red  color  to  the  deep  soil  of  the  valley.  The  west-facing  slope, 
below  the  Dakota  crest,  is  the  outcrop  of  a calcareous  sandstone 
stratum  which  is  weathered  so  slowly  as  to  be  covered  only  by  a 
thin  soil.  In  certain  places  this  limy  sandstone  stratum  is  hard 
enough  to  form  a separate  ridge  or  hogback  crest. 

The  Dakota  hogback  is  one  of  the  most  constant  and  conspicu- 
ous topographic  features  of  the  mountain-front,  since  it  is  practically 
everywhere  harder  than  the  strata  above  and  below.  Its  top  is 
usually  quite  even  and  straight,  representing  the  level  of  a former 
graded  surface.  Its  crest  is  quite  rocky;  there  is  no  soil  except  in 
the  crevices. 

The  present  graded  slope  to  the  plains  begins  usually  with  the 
outer  slope  of  the  Dakota  hogback,  through  first  a layer  of  dark 
shales,  then  a thin  limestone  overlaid  by  soft  light-colored  shales, 
then  clays  and  shales.  Near  every  east-flowing  stream,  however, 
the  graded  slope  is  likely  to  be  cut  beneath  by  side-gulches  cutting 
down  into  the  dark  shales,  leaving  a cut-off  mesa  with  the  limestone 
at  its  high  western  end. 

Local  distribution  of  vegetation  in  the  mountain-front  belt  of 
upturned  sedimentary  rocks  presents  a variability  apparently 
dependent  almost  entirely  upon  topography  and  soil  texture,  just 
as  in  the  area  of  granitic  foothills.  There  seem  to  be  few  if  any 
perceptible  differences  in  the  floras  of  the  different  geological 
formations  which  can  be  traced  to  chemical  differences  in  the  sub- 
stratum. It  is  perhaps  true  that  cedars  are  more  frequent  in  the 
limestone  or  calcareous  sands  of  the  stratum  just  below  the  Dakota, 
where  these  are  exposed  in  gulches  which  notch  the  Dakota  hog-  - 
backs,  and  that  there  are  certain  slight  floristic  differences  between 


igig]  VESTAL— PHYTOGEOGRAPHY  OF  COLORADO  165 

granitic  and  sedimentary  areas.  This  question  has  been  discussed 
by  Ramaley  (9),  who  found  the  two  areas  about  the  same  in 
floras  (in  the  Poudre  mountain-front  area),  with  Cercocarpus 
abundantly  represented  in  the  sandstone  but  not  in  granite, 
Selaginella  apparently  absent  from  the  sandstone,  and  lichens 
infrequent  there.  Following  a suggestion  from  Cowles,  it  appears 
to  the  writer  that  differences  in  rate  of  erosion  of  the  substratum 
may  explain  the  distribution  of  lichens,  and  perhaps  Selaginella 
also.  The  sandstones  are  rather  soft  in  the  Poudre  area,  and  wear 
away  too  rapidly  for  the  lichens  to  establish  themselves  abundantly. 
The  Fountain  sandstone  is  harder  in  the  Boulder  region  than  else- 
where, and  there  at  least  it  bears  lichens  almost  as  abundantly  as 
do  the  granites.  Selaginella  is  frequent  in  the  sedimentary  rocks  in 
the  Boulder  area,  as  Ramaley  has  pointed  out.  The  writer  knows 
of  no  plants  which  are  restricted  to  either  sedimentary  or  granitic 
areas,  the  only  observed  differences  being  those  of  relative  abun- 
dance. The  gulches,  exposed  slopes  and  crests,  etc.,  of  the  sedimen- 
tary area  are  quite  comparable  to  similar  topographic  situations  of 
the  granitic  foothills,  and  have  practically  the  same  plant  assem- 
blages. 

The  rocky  upper  slopes  of  the  Fountain,  the  Dakota,  and  other 
ridge-making  strata,  where  they  occur,  lack  soil  except  in  crevices, 
and  are  mostly  bare,  except  where  rock  pines  or  pinyons,  shrubs  of 
rocky  situations  ( Cercocarpus , Ribes,  Jamesia,  etc.),  and  crevice 
plants,  including  many  xerophytic  herbs,  can  obtain  a foothold. 
The  west  slope  of  hogbacks  is  blufflike,  usually,  and  rocky,  while 
the  east  slope  is  less  steep  (depending  on  the  local  angle  of  dip) 
and  likely  to  be  strewn  with  debris,  as  are  the  slopes  of  the  harder 
exposures  of  the  valley,  and  these  have  shrubby  or  herbaceous 
vegetation,  sparse,  and  of  species  of  rocky  situations.  The  softer 
shales  occupying  the  bottom  of  the  valley  are  usually  deeply  buried 
by  debris  (of  fine  soil  with  imbedded  rock  fragments  of  all  sizes), 
and  support  a grassland  vegetation,  which  is  luxuriant  in  the 
rainier  parts  of  the  growing  season  and  very  dry  the  rest  of  the  time. 
A stream-bed  in  the  bottom  of  the  valley  may  be  bordered  by  a 
strip  of  mixed  shrub,  Crataegus , oak,  or  canyon  forest;  or  if  dry, 
by  scattered  narrow-leaf  cottonwoods  and  willows.  Mesophytic 


i66 


BOTANICAL  GAZETTE 


[SEPTEMBER 


ravines  developing  in  the  sedimentary  area  support  mixed-shrub, 
woodland,  or  mesophytic  herbaceous  growths,  as  in  the  granitic 
foothills.  Local  meadows  (mesophytic  grassland)  are  found  on 
slopes  where  seepage  or  a high  water  table  moistens  a deep  soil  for 
at  least  part  of  the  growing  season. 

In  places  the  sedimentary  rocks  have  been  worn  down  more  than 
is  common,  so  that  they  are  mostly  or  in  part  reduced  to  a general 
grade,  above  which  the  more  resistant  layers  rise  locally.  This 
is  the  condition  in  the  valleys  of  some  of  the  larger  streams  from 
the  foothills,  and  is  seen  at  Platte  Canyon,  partially  at  Bear  Creek 
(Morrison),  and  also  in  Perry  Park  (fig.  4)  and  the  Garden  of  the 
Gods.  The  floor  of  this  graded  surface,  especially  in  the  Fountain 
exposures,  is  likely  to  be  covered  very  thinly  with  small  angular 
fragments,  loose  or  compacted.  The  vegetation,  as  well  as  the 
soil,  is  very  like  that  of  gravelly  floors  in  the  foothills,  being  a 
variant  of  the  primitive  grassland  association,  with  scattered  rosette 
or  mat  plants,  Bouteloua  hirsuta,  etc. 

The  climatic  transition  in  the  zone  of  upturned  sedimentary 
strata  is  rapid.  At  Boulder  and  elsewhere  dense  cloud-banks 
have  frequently  been  seen  to  descend  to  or  just  beneath  the  Foun- 
tain crags  without  continuing  outward  and  downward  to  the 
plains  (figs.  6,  7).  The  outer  granitic  hills  and  upper  sedimentary 
slopes  receive  greater  and  more  frequent  precipitation  than  the 
lower  slopes  and  adjacent  mesas  and  plains;  it  may  rain  slightly 
below  while  it  snows  considerably  above  (cf.  fig.  12);  the  outer 
and  lower  slopes  are  more  exposed  to  wind,  less  cloudy,  and  in 
places  less  shaded  from  the  afternoon  sun  by  the  higher  granitic 
hills  than  the  inner  valleys  and  upper  slopes.  No  exact  data  are 
available  for  this  sudden  climatic  transition.  Where  the  outcrop 
of  sedimentary  ridges  and  valleys  is  wide,  as  in  the  northern 
mountain-front  region,  the  outer  hogbacks  are  severely  exposed 
to  sun  and  wind,  as  in  the  open  plains.  Their  coarse  rocky  soil 
favors  woody  plants ; the  xerophytic  Cercocarpus  shrub  assemblage 
is  here  more  extensively  developed  than  anywhere  else. 

Mesas  and  graded  slopes  of  the  debris-apron  (fig.  8). — 
The  general  character  of  the  graded  slopes  and  their  mesa-fragments 
has  already  been  suggested.  The  mesas  are  of  varying  ages  and 


1919]  VESTAL— PHYTOGEOGRAPHY  OF  COLORADO  167 

levels.  They  are  described  in  the  accounts  of  Lee  (6),  Johnson  (5), 
Fenneman  (2),  Shantz  (15),  Ramaley,  Robbins,  and  Dodds  (12), 
and  Vestal  (17).  The  topographic  parts  of  a mesa  are:  (1)  the 


Figs.  6,  7. — Climatic  transition  at  mountain-front:  fig.  6,  outer  mountains  just 
south  of  Boulder,  seen  from  university  campus;  clouds  beginning  to  form  at  summit 
of  Green  Mountain,  while  much  of  South  Boulder  Peak,  at  extreme  left,  is  already 
obscured;  snow  covers  the  mountain  slopes  and  fades  out  toward  base  of  high  mesas; 
roofs  of  distant  buildings  also  white;  fig.  7,  practically  same  view,  a little  later,  with 
upper  slopes  obscured;  at  one  time  it  began  to  snow  on  mountains  and  upper  mesas, 
and  a few  minutes  later  to  rain  in  town;  shortly  afterward  it  changed  to  snow  in  the 
upper  edge  of  town,  so  that  the  roof  of  the  building  with  the  short  steeple  at  the  right 
in  midground,  and  of  nearer  houses,  were  well  whitened,  while  rain  still  fell  on  the 
campus,  less  than  half  a mile  away,  and  not  more  than  50  ft.  lower;  difference  in  ele- 
vation at  mountain-front  is  critical  as  regards  climatic  change. 


mesa-top,  with  flat  surface  covered  with  mixed  rock-debris;  (2)  the 
edge  or  mesa-crest;  (3)  the  side-slope;  and  (4)  valleys  or  draws  in 
the  side-slope.  The  soil  conditions  and  their  effects  on  plant  dis- 
tribution have  been  discussed  in  the  three  articles  last  cited. 


i68 


BOTANICAL  GAZETTE 


[SEPTEMBER 


The  debris-cover,  where  it  has  not  been  removed  by  recent 
erosion,  extends  far  out  into  the  plains.  Its  removal  from  the 
extensive  areas  of  soft  shales  and  clays  marks  a change  from  the 
flat  terrace  level  to  the  easily  eroded,  gently  rolling  surface  of  much 
of  the  plains.  The  High  Plains  are  extensive  remnants  of  the  old 
graded  surface,  away  from  the  mountains. 

The  north-south  distribution  of  the  terraces  is  practically  that 
of  the  mountain-front,  although  as  conspicuous  topographic  forms 
the  mesas  are  not  so  extensive.  So  far  as  effects  on  distribution  of 
vegetation  are  concerned,  the  presence  of  the  coarse  mixed  soil  of 


Fig.  8. — Table  Mesa,  about  7 miles  north  of  Boulder;  outlines  of  hills  sketched 
in  with  ink;  ridge  DDD  is  Dakota  hogback;  Boulder  mesas  may  be  seen  in  figs.  6 
and  7. 


the  detrital  surfaces  is  the  important  physical  condition.  It 
permits  the  growth  in  the  same  small  area  of  a rich  variety  of  plants, 
representing  numerous  vegetation-types  and  different  geographic 
elements. 

Plateau  areas,  buttes,  and  escarpments  (figs.  9-1 1). — 
Where  the  sedimentary  strata  are  horizontal  or  of  rather  slight  dip 
the  harder  layers  protect  the  softer  rocks  beneath,  and  extensive 
plateau  surfaces  are  left  above  the  grade  established  by  present 
erosion.  These  can  be  invaded  only  at  the  edge  and  by  ravines 
which  eat  their  way  headward  into  the  bluffs.  Smaller  elevated 
areas  or  buttes,  recently  or  long  ago  cut  away  from  plateaus  by 


VESTAL— PHYTOGEOGRAPHY  OF  COLORADO 


1919] 


169 


meeting  of  two  such  ravines,  are  common.  Older  buttes  are 
fewer  and  more  distant  from  one  another. 


Figs.  9,  10. — Buttes  and  plateau  areas:  fig.  9,  North  Table  Mountain  at  Golden, 
west  of  Denver;  this  and  South  Table  Mountain  are  capped  with  basalt;  fig.  10, 
Fisher  Peak,  northern  end  of  Raton  mesas,  as  seen  from  valley  of  Purgatoire  River, 
a few  miles  above  Trinidad;  upland  in  midgtound  belongs  to  southern  sedimentary 
plateau;  vegetation  is  principally  dry  grassland  with  scattered  pinyons  and  cedars 
and  infrequent  clumps  of  scrub  oak. 

The  plateaus  and  buttes  are  found  outside  of  the  upturned  ridge 
and  valley  zone  wherever  the  surface  rocks  are  rather  resistant. 
These  resistant  strata  are  usually  the  most  recent  and  uppermost, 


BOTANICAL  GAZETTE 


[SEPTEMBER 


170 

although  the  much  older  Dakota  is  at  the  surface  over  consider- 
able areas  in  the  plains  drained  by  the  southern  tributaries  of  the 
Arkansas.  For  a considerable  thickness  above  the  Dakota  the 
strata  are  mostly  soft  shales,  which  erode  too  readily  to  give  table- 
land topography. 

The  larger  through  streams  and  their  tributaries  have  cut  below 
the  level  of  the  High  Plains,  leaving  escarpments  which  are  par- 
ticularly notable  near  the  Platte-Arkansas  divide.  Plum  and 


Fig.  ii.— Buttes  and  plateau  areas:  divide  between  East  Plum  and  West  Plum 
creeks,  in  Castle  Rock  area,  showing  some  of  rhyolite  buttes;  one  of  the  most 
imposing  of  these,  Dawson  Butte,  shown  in  fig.  4. 


Cherry  creeks,  running  north  into  the  Platte  from  the  divide,  and 
Monument,  running  into  Fountain  Creek,  south  to  the  Arkansas, 
have  eroded  deep  valleys  parallel  to  the  mountain-front.  Away 
from  the  mountain-front  proper  these  valleys  are  bounded  by  lines 
of  steep  cliffs,  but  the  west  border  of  Monument  and  West  Plum 
Creek  valleys  is  the  graded  slope  from  the  foothills,  with  its  debris- 
covered  terraces.  Isolated  buttes  are  present  within  these  valleys, 
some  of  them  protected  by  caps  of  igneous  rocks  from  local  outflows. 

The  southern  part  of  the  Sangre  de  Cristo  Range  (sometimes 
considered  as  a separate  mountain  chain,  the  Culebra  Range)  is 
flanked  on  the  east  by  a sedimentary  plateau  which  rises  abruptly 
above  the  plains  in  a steep  line  of  bluffs.  The  plateau  is  of  sand- 
stones mostly,  of  slight  dip,  and  is  much  dissected  by  the  eastward 


1919]  VESTAL— PHYTOGEOGRAPHY  OF  COLORADO  17 1 

flowing  streams  and  their  tributaries.  On  it  rests  the  highland  of 
the  Spanish  Peaks,  and  it  is  ribbed  by  resistant  dikes  of  igneous 
material  from  two  outflows,  one  set  radiating  downward  from  the 
peaks  themselves.  With  the  plateaus  should  be  classed  the  high 
lava-capped  mesas  of  the  mountain-front  and  plains  in  the  area 
near  the  Colorado-New  Mexico  boundary. 

As  in  the  hogback  ridges,  vegetation  distribution  in  the  plateau 
and  butte  areas  is  largely  determined  by  soil  texture  and  topog- 
raphy. Atmospheric  conditions  vary  with  exposure  to  wind  and 
sun.  The  tops  of  the  plateaus  are  covered  with  short-grass  and 


Fig.  12. — Unbroken  short-grass  ground  cover  in  plains 


mixed  grassland  over  the  level  upland  stretches  of  comparatively 
fine-textured  soil.  Exposed  cliffs  and  crests,  and  rocky  debris- 
slopes,  afford  lodging  places  for  woody  xerophytes  ( Cercocarpus , 
rock  pines,  pinyons,  and  cedars),  with  primitive  grassland  as  the 
general  ground-cover.  The  deeper  and  shaded  parts  of  canyons 
and  ravines  approach  a mesophytic  condition,  with  mixed  shrub 
and  woodland  vegetation. 

PLAINS 

Plains  topography  is  typically  flat  or  gently  rolling  country, 
with  fine  clay  soil  from  a soft-shale  substratum.  Short-grass  is 
the  characteristic  vegetation  (fig.  12).  Where  the  substratum  is 


172 


BOTANICAL  GAZETTE 


[SEPTEMBER 


sandstone  the  soil  is  more  porous,  with  much  sand;  and  plants 
of  an  assemblage  typical  of  sandy  soil  are  seen  (17).  Sand  hills  are 
present  locally,  usually  to  the  leeward  of  larger  streams. 

Near  the  mountains  the  debris-cover,  if  present,  considerably 
modifies  topography,  soil  conditions,  and  vegetation.  It  may 
extend  a long  way  into  the  plains,  or  may  have  been  removed  very 
near  the  beginning  of  the  graded  slope  from  the  foothills. 

Saline  or  alkaline  areas  are  locally  present.  The  valleys  of  the 
Arkansas  and  its  tributaries  (wet-weather  streams,  many  of  them, 
with  trenched  flood-channels)  are  in  many  places  alkaline,  and  show 
prominent  stands  of  Sarcobatus-Chrysothamnus  vegetation. 

Woody  vegetation  from  the  foothills  extends  locally  far  into  the 
plains  in  rock  outcrops,  and  along  stony  crests  of  stream-bluffs  or 
terraces.  The  larger  streams  are  bordered  for  many  miles  from 
the  mountains  by  cottonwoods,  usually  scattered. 

Climate 

The  region  has  a continental  climate,  semi-arid,  less  so  at  the 
base  of  the  mountains  and  in  the  foothills,  with  most  of  the  rainfall 
in  the  warmer  months.  Wind  movement,  proportion  of  sunshine, 
and  evaporating  power  of  the  air  are  high  in  the  plains,  with  wide 
extremes  of  temperature;  all  of  these  features  are  less  marked  in 
the  foothills. 

The  southern  part  of  the  region  is  warmer  and  drier  than  the 
northern,  and  with  different  distribution  of  rainfall.  The  rapid 
east-west  change  in  elevation  and  topography  at  and  near  the 
mountain-front  is  accompanied  by  more  or  less  considerable  climatic 
variation;  this  with  the  local  peculiarities  occasioned  by  the  ele- 
vated Platte-Arkansas  divide,  and  the  differences  between  areas 
north  and  south  of  the  divide,  may  be  seen  in  the  summaries  of 
climatic  data  for  the  particular  subregions.  These  data  have  been 
taken  from  the  summary  of  Climatological  Data  for  eastern  Colo- 
rado, southeastern  Wyoming,  and  northeastern  Colorado.4  The 
facts  shown  in  table  II  should  be  considered  in  the  light  of  their 

4 Section  6,  northeastern  New  Mexico,  by  C.  E.  Linney.  Section  7,  region  drained 
by  the  Arkansas  in  Colorado,  and  section  8,  region  drained  by  the  Platte  in  Colorado, 
by  F.  H.  Brandenburg.  Section  24,  southeastern  Wyoming,  by  W.  S.  Palmer. 


igig]  ' VESTAL— PHYTOGEOGRAPHY  OF  COLORADO  173 

determinative  influence  upon  the  vegetation;  this  can  be  done  in 
only  the  barest  manner  in  this  section,  but  these  relations  are  again 
brought  out  in  the  part  on  geographic  relations  of  the  vegetation. 

Temperature  conditions  of  the  different  parts  of  the  region  may 
be  summarized  as  follows:  The  foothills  have  a lower  mean  tem- 
perature and  shorter  period  without  frost  than  either  plains  or 
mountain-front.  Certain  of  the  foothills  vegetation-types  and 

TABLE  II 
Temperature  data 


Area 

Average  mean 
temperature  0 F. 

Maximum 

temperature 

Minimum 

temperature 

Average  number 
of  days  in 
growing  season 

Foothills  (4) 

43  - 1 

100 

-36 

99 

Northern  (2) 

42.0 

98 

-32 

95 

Southern  (2) 

44-3 

100 

-36 

104 

Mountain-front  (5,  excl. 
Divide) 

50.5 

104 

-30 

154 

Northern  (1,  Boulder).. . . 

50.9 

97 

— 20 

164 

' Divide  (2) 

46.7 

99 

“33 

122 

Southern  (4) . . , 

50-3 

104 

-30 

151 

Plains  near  mountains  (5) . . 

47-7 

105 

-38 

138 

Northern  (3) 

47-i 

io5 

-38 

134 

Southern  (2) 

48.6 

103 

-32 

143 

Dry  plains  (5) 

50.4 

106 

-45 

151 

Northern  (2) 

48.6 

103 

-45 

•145 

Southern  (3) 

51.0 

106 

-32 

156 

“Northern  area”  (8) 

45-8 

105 

— 45 

131 

“Southern  area”  (n) 

49  3 

106 

-36 

142 

The  number  of  stations  for  each  area  is  given  in  parentheses.  The  mountain-front  does  not  include 
the  two  stations  of  the  Platte-Arkansas  divide,  which  is  so  much  more  elevated  than  other  parts  of  the 
mountain-front  as  to  be  much  cooler.  The  “northern  and  southern  areas”  are  respectively  the  northern 
and  southern  parts  of  the  region,  each  extending  over  foothills,  mountain-front,  and  plains. 


many  of  the  plant  species  are  characteristic  of  northeastern  and 
northwestern  coniferous  forest  regions,  are  in  fact  southern  exten- 
sions of  them.  The  boreal  character  is  much  more  evident  in  the 
higher  mountains  than  in  the  foothills. 

The  mountain-front  has  the  longest  frostless  season,  the  highest 
mean  temperature,  the  mildest  winters,  and  the  least  range  in 
temperature  extremes.  Mountain-front  localities  are  mostly 
comparatively  sheltered;  temperature  inversion  is  common. 
Early  spring  plants  flower  several  weeks  earlier  at  the  mountain- 
front  than  in  either  plains  or  foothills;  at  Boulder  in  spring  the 


174 


BOTANICAL  GAZETTE 


[SEPTEMBER 


season  is  in  general  2-3  weeks  in  advance  of  that  of  Denver,  14 
miles  from  the  mountains. 

The  divide  between  Platte  and  Arkansas  drainage,  which  should 
be  considered  in  connection  with  the  mountain-front  area,  has  a 
mean  temperature  and  frostless  period  intermediate  between  those 
of  mountain-front  and  foothills  areas,  as  it  is  intermediate  in  alti- 
tude and  in  vegetation. 

The  plains  have  a slightly  lower  mean  temperature  and  shorter 
season  without  frost  than  the  mountain-front  area;  the  tempera- 
ture of  the  dry  plains  at  some  distance  from  the  mountains 
approaches  that  of  the  mountain-front  more  closely  than  that  of 
the  plains  adjoining  it.  This  difference  is  accompanied  by  a 
floristic  one.  Temperature  extremes  are  greatest  in  the  plains,  a 
condition  inimical  to  growth  of  woody  plants. 

The  plains,  mountain-front,  and  foothills  in  northern  Colorado 
(“northern  area”)  are  cooler  than  those  to  the  south,  but  the  north- 
south  differences  in  temperature  and  length  of  growing  season  due 
to  latitude  are  of  much  smaller  range  and  influence  upon 
vegetation  than  the  east-west  differences  due  to*  altitude  and 
changes  of  topographic  character. 

For  purposes  of  comparison  table  III  includes  rainfall  data  for 
the  higher  parts  of  the  mountains  bordering  the  foothills  on  the 
west  (montane  zone) , and  for  the  plains  of  eastern  Colorado  border- 
ing the  region  studied  on  the  east.  Annual  rainfall  is  higher  to  the 
west,  increasing  with  elevation,  and  higher  also  in  the  eastern 
plains,  as  a part  of  the  gradual  geographic  increase  of  rainfall  from 
the  dry  belt  of  the  Great  Plains  eastward  through  the  prairie  region 
to  the  border  of  the  eastern  deciduous  forest  region.  The  eastern 
plains  mark  the  transition  from  short-grass  plains  to  the  taller 
prairie-grass  vegetation  of  the  prairie,  and  are  known  in  Colorado 
as  “ the  rain  belt.”  The  driest  part  of  the  plains  region  lies  between 
the  rain  belt  and  the  plains  near  the  mountains,  in  a zone  distant 
from  the  mountains  about  18-25  miles,  and  of  a breadth  30-60 
miles.  It  is  narrowed  on  the  west  by  the  elevation  of  the  Platte- 
Arkansas  divide,  and  extends  farther  eastward  in  the  Arkansas 
River  Valley.  It  extends  only  a little  way  north  into  Wyoming 
and  apparently  is  much  narrowed  on  the  west  in  extreme  southern 


Rainfall  data 


VESTAL— PHYTOGEOGRAPHY  OF  COLORADO 


I 


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The  northern  area  is  the  aggregate  of  northern  foothills,  mountain-front,  and  plains;  the  southern  area  is  its  southern  equivalent.  The  number  of  stations  is 
given  in  parentheses  for  each  area,  and  the  range  of  altitude  above  sea-level  of  the  several  north-south  zones  (in  feet)  is  also  included.  The  rainfall  figures  are 
in  inches. 


176 


BOTANICAL  GAZETTE 


[SEPTEMBER 


Colorado  and  northeastern  New  Mexico  by  the  lava-capped  plateaus 
which  there  extend  eastward  from  the  mountains. 

As  for  the  mountain-front  and  adjacent  foothills  and  plains, 
the  first  two  average  about  the  same,  the  mountain-front  receiving 
slightly  more  rainfall  in  the  north  and  at  the  divide  than  the  foot- 
hills. This  may  perhaps  be  due  to  the  fact  that  in  the  northern 
part  of  the  state,  as  at  Boulder,  the  rain  usually  comes  with  east  and 
northeast  winds;  and  since  the  change  of  elevation  is  greatest  at 
the  mountain-front,  more  precipitation  might  occur  there  than  in 
the  foothills  beyond.  At  Boulder  the  more  mesophytic  forms  of 
vegetation  occur  more  frequently  and  in  larger  areas  in  the  sedi- 
mentary rocks  of  the  mountain-front  than  in  the  granite  foothills 
half  a mile  or  a mile  inside  the  foothills.  In  the  southern  part  of  the 
state  the  mountain-front  .is  drier  than  the  foothills,  as  a rule.  The 
plains  near  the  mountains  receive  almost  2 inches  less  rain,  on 
the  average,  than  foothills  and  mountain-front,  and  the  dry 
plains  to  the  east  ne'arly  another  2 inches  less. 

The  “ northern  area”  (foothills,  mountain-front,  and  adjacent 
plains)  receives  on  the  average  about  1 inch  greater  rainfall  than 
the  “southern  area.”  Coupled  with  the  higher  temperature  and 
greater  evaporation,  this  results  in  a considerably  more  xerophytic 
vegetation  south  of  the  Platte-Arkansas  divide.  There  is  little 
difference  in  the  plains,  but  at  the  mountain-front,  with  a difference 
of  1.57  inches,  the  vegetation  to  the  south  is  markedly  drier. 

Cooper  finds,  in  the  chaparral  region  of  California,  that  very 
slight  differences  in  the  original  physical  conditions  of  north  and 
south  slopes  result  in  very  marked  differences  in  -vegetation.  The 
same  principle  seems  to  apply,  in  perhaps  a smaller  degree,  in  a 
semi-arid  region  like  the  Colorado  mountain-front.  It  appears  that 
differences  in  rainfall  from  place  to  place,  or  from  month  to  month, 
although  small  in  absolute  amount,  can  be  critical  in  their  influence 
upon  vegetation  distribution.  The  slight  differences  appear  to 
represent  marginal  values  above  or  below  a critical  point.  The 
difference  in  vegetation  in  two  areas,  moreover,  is  not  necessarily 
the  result  of  climatic  difference,  but  is  a resultant  of  differences  in 
soil,  topography,  geographic  position,  and  vegetational  history,  in 
addition  to  climate.  It  should  not  be  surprising,  therefore,  that 


1919] 


VESTAL— PHYTOGEOGRAPHY  OF  COLORADO 


177 


areas  having  climates  not  widely  dissimilar,  as  the  plains  of  the 
rain  belt  and  the  northern  foothills,  should  have  distinctly  unlike 
vegetation. 

Minimum  rainfall. — One  factor  which  seems  to  be  partly 
responsible  for  the  generally  xerophytic  character  of  the  entire 
region  studied,  the  plains  in  particular,  is  the  wide  variation  in  the 
amount  of  rainfall  from  year  to  year.  The  minima  have  been 

TABLE  IV 

Minimum  annual  rainfall 


Area 

Number  of 

STATIONS  WITH 

RECORDS 

Average  minimum 

FOR  AREA 

Lowest  minimum  ; 

STATION 

RECORDED  FOR  ANY 

IN  AREA 

1893 

Other 

years 

1893 

Other 

years 

1893 

Other  years 

Montane  zone 

- 

6 

15  65 

16.55 

Frances 

11.36  (1907) 
Cripple  Creek 

Foothills 

Mountain-front  and 

4 

9 

12.83 

7. 16 

Box  Elder 

10.93  (i9°8) 
Cheesman 

divide 

7 

11 

9.12 

II  .91 

7 03 

Waterdale 

8.76  (1890) 
Table  Rock 

Plains  near  mountains . 

5 

6 

9-39 

8.91 

7. n 

Fort  Collins 

5.04  (1876) 
Cheyenne 

Dry  plains  belt 

7 

7 

8. 11 

7.01 

5-40 

Greeley 

3.78  (1894) 
Las  Animas 

Eastern  plains 

5 

8 

10.48 

IO.61 

8.30 

Cheyenne  Wells 

6.97  (1894) 
Cope 

Two  stations  within  the  foothills  area  are  exceptional  as  to  rainfall,  and  have  not  been  included  in  the 
averages.  These  are  Salida  in  the  Arkansas  Valley  above  the  Royal  Gorge,  and  Westcliffe  in  the  Wet  Moun- 
tain Valley.  Similarly,  Canyon  City  at  the  debouchure  of  the  Arkansas,  and  Raton  and  Las  Vegas  in  New 
Mexico,  have  been  excluded  from  the  mountain-front  area.  The  stations  with  the  lowest  minima  have  been  men- 
tioned in  the  table.  The  lowest  minimum  in  each  area,  whether  in  1893  or  in  some  other  year,  is  printed  in 
bold  face.  Except  for  Cheyenne  Wells,  which  is  remote  from  the  mountains,  all  of  the  stations  noted  as  having 
had  least  rainfall  in  1893  are  within  a limited  area  (in  the  northern  part  of  the  region),  which  seems  to  have  been 
most  severely  affected  by  the  drought  of  that  year. 


tabulated  for  the  several  parts  of  the  region  from  the  climatic 
summaries  of  the  Weather  Bureau.  The  year  1893  happened  to 
be  exceptionally  dry,  and  the  minima  for  many  of  the  stations  fall 
in  it.  Dryness  in  other  years  has  been  of  more  local  prevalence. 
It  has  seemed  preferable  to  present  the  data  for  1893  separately 
from  that  of  other  years.  The  data  for  1893  are  not  available  for 
all  stations  in  each  area,  and  so  the  number  of  stations  from  which 
data  have  been  used  is  mentioned  for  each  area  (table  IV).  The 
column  presenting  the  average  minima  for  the  several  areas  (minima 


178 


BOTANICAL  GAZETTE 


[SEPTEMBER 


of  all  stations  for  each  area  averaged,  excluding  figures  for  1893) 
seems  to  express  the  main  fact  of  the  table,  that  the  rainfall  reaches 
successively  lower  minima  downward  and  eastward  from  the 
mountain  zone  through  the  foothills,  mountain-front,  and  adjacent 
plains  to  the  dry  plains  belt,  beyond  which  the  minima  rise  gradually 
with  the  gradual  increase  of  rainfall  eastward  into  the  prairie- 
grass  region.  It  appears  also  that  annual  rainfall  values  falling 
considerably  below  the  average  (as  much  as  4 inches  below)  occur 
more  frequently  in  the  plains  than  in  the  mountain-front  and  foot- 
hills areas.  The  well  known  uncertainty  of  farming  without  irri- 
gation in  much  of  eastern  Colorado,  due  to  frequency  of  very  dry 
years,  indicates  further  that  it  is  not  the  average  rainfall  so  much 
as  the  constantly  recurring  minimum  which  determines  whether 
or  not  an  area  can  support  a cultivated  or  natural  vegetation  which 
is  other  than  decidedly  xerophytic. 

Seasonal  distribution  of  rainfall. — On  the  whole,  precipi- 
tation during  the  cooler  months  is  quite  low ; this  is  not  so  true  of  the 
montane  area  just  to  the  west  of  and  above  the  foothills.  The 
summer  rainfall  is  greater,  but  in  most  places  distributed  rather 
unevenly.  June  is  thus  drier  than  either  May  or  July  over  prac- 
tically the  entire  region.  The  northern  area  near  the  mountain- 
front  receives  more  rain  in  the  spring  and  early  summer  months, 
while  the  southern  area  receives  more  of  its  rain  during  late  summer. 
This  difference  between  north  and  south  is  of  far-reaching  influence 
upon  the  character  and  distribution  of  vegetation.  The  details 
of  seasonal  distribution  of  rainfall  are  shown  in  the  table  of  averages 
of  rainfall  data,  and  in  figs.  13-16. 

The  northern  and  southern  parts  of  the  zones  at  and  near  the 
mountain-front  are  so  different  as  to  rainfall  that  they  cannot  be 
incorporated  in  single  graphs.  The  northern  parts  of  the  zones 
are  selected,  therefore,  as  the  more  typical.  The  graph  for  the 
mountain-front  is  omitted  to  avoid  overcrowding,  but  it  can  be 
seen  in  fig.  14.  Excluding  the  eastern  plains,  the  zones  have  the 
same  type  of  rainfall,  with  greatest  abundance  in  May  and  July, 
and  a decline  in  June.  The  zones  are  successively  drier  with 
decrease  of  elevation,  and  this  is  almost  as  true  for  particular 
months  as  it  is  for  the  entire  year.  The  eastern  plains  have  higher 


1919]  VESTAL— PHYTOGEOGRAPHY  OF  COLORADO  179 

summer  rainfall  than  the  plains  near  the  mountains;  the  distribu- 
tion is  similar,  except  that  June  is  as  rainy  as  May. 

The  graphs  for  foothills  and  plains  near  the  mountains  are 
repeated  in  fig.  14.  These  zones  and  the  mountain-front  have 
maximum  rainfall  in  May,  with  a sharp  decline  in  June,  followed 
by  slightly  greater  rainfall  in  July.  Despite  its  less  elevated  posi- 
tion, the  mountain-front  receives  greater  spring  rainfall  than  the 
foothills. 


Figs.  13,  14. — Seasonal  distribution  of  rainfall:  fig.  13,  comparison  of  north-soyth 
zones;  fig.  14,  northern  foothills,  mountain-front,  and  plains. 


The  data  for  the  south  are  not  so  dependable  as  for  the  north, 
for  some  of  the  few  stations  are  exceptionally  situated.  The 
contrast  shown  with  the  northern  area  is  marked,  however.  The 
rain  is  less  abundant  in  spring  and  more  abundant  in  July  and 
August  than  to  the  north. 

The  graph  shown  for  the  “ northern  area”  is  a composite  of  the 
3 in  fig.  14,  that  of  the  “southern  area”  is  a composite  of  those 


i8o 


BOTANICAL  GAZETTE 


[SEPTEMBER 


in  fig.  15;  they  contrast  strongly.  The  northern  area  is  character- 
ized by  the  Rocky  Mountain  foothill  type  of  rainfall,  the  southern 
area  by  the  New  Mexican  type  (Ward  19).  Both  of  these  types 
are  described  as  having  a single  maximum,  for  the  first  in  May,  for 
the  second  in  July- August.  The  northern  area  receives  most  of 
its  rain  from  northeasterly  winds;  the  southern  area  probably 
from  southeasterly  winds.  The  centrally  situated  Platte-Arkansas 

k 


J 

^ , 

:/a 

// 

# 

/ / 

;// 

;// 

\ /V 

/ 

\ 

-Southern  Foothills  -Southern  Mountain-Front  Southern  Plains 


Figs.  15,  16. — Seasonal  distribution  of  rainfall:  fig.  15,  southern  foothills, 
mountain-front,  and  plains;  fig.  16,  northern  and  southern  areas  and  divide. 


divide  receives  rain  from  both  directions,  and  has  both  maxima, 
with  a higher  June  rainfall,  partly  because  of  its  considerable  ele- 
vation. Probably  rain  is  carried  from  either  direction  past  the 
divide,  producing  secondary  maxima,  in  July  in  the  northern  area, 
and  in  April  in  the  south. 

The  abundant  rainfall  of  the  divide,  especially  in  June,  forms  a 
local  rainfall  type  which  is  intermediate  between  that  of  well 
watered  parts  of  the  foothill  zone  and  that  of  the  eastern  or  rain- 


1919] 


VESTAL— PHYTOGEOGRAPHY  OF  COLORADO 


181 


belt  plains.  The  divide  is  also  cooler  than  most  parts  of  the 
mountain-front  and  adjoining  plains.  The  vegetation  of  the  divide 
is  likewise  transitional  between  that  of  rain  belt  and  foothills, 
with  the  more  nearly  mesophytic  forms  of  grassland,  and  with 
woody  plants  of  the  foothills  extending  many  miles  eastward  from 
the  mountains. 

The  effects  upon  the  vegetation  of  the  difference  in  distribution 
of  precipitation  north  and  south  of  the  divide  are  discussed  in 
the  section  on  geographic  relations,  but  may  be  summarized  briefly 
herewith. 


TABLE  V 

Influence  of  seasonal  distribution  of  rainfall  on  vegetation 


NORTHERN  AREA 

The  greatest  rainfall  is  in  April  and 
May. 

There  is  greater  activity  of  vege- 
tation, more  luxuriant  growth,  and 
greatest  abundance  of  flowers  in 
spring. 

There  are  many  spring- flowering 
plants  from  the  mountains  of  rather 
mesophytic  character,  in  mixture  with 
plains  plants  in  the  mountain-front 
kone. 

Distribution  of  the  bunch-grass 
association  and  of  the  less  xerophytic 
plants,  requiring  a long  season  of  con- 
tinued moisture,  is  limited. 

The  northern  plains  near  the 
mountain-front  flower  luxuriantly  in 
spring  and  early  summer,  but  only 
the  more  xerophytic  composites,  etc., 
in  late  summer,  in  which  respect  the 
plains  are  more  like  the  driest  plains 
just  east  of  them  in  late  summer. 


SOUTHERN  AREA 

The  greatest  rainfall  is  in  July  and 
August. 

There  is  greater  activity  of  vege- 
tation and  more  luxuriant  growth  in 
late  summer. 

There  is  absence  or  scarcity  of 
spring-flowering  mountain  plants,  and 
greater  prevalence  of  plains  plants  in 
the  mountain-front  zone. 

Distribution  of  bunch-grass  is  less 
restricted;  there  is  a greater  preva- 
lence of  late-flowering  plants  not 
intensely  xerophytic,  as  some  of  the 
asters  and  goldenrods,  etc. 

The  southern  plains  near  the  moun- 
tains contain  fewer  spring  flowers, 
but  many  long-season  plants  absent 
from  the  dry  plains  and  the  northern 
plains  near  the  mountains  are  present, 
as  the  annual  sunflowers.  In  this 
respect  the  plains  are  more  like  those 
of  the  rain  belt  of  eastern  Colorado 
in  late  summer. 


It  is  remembered  that  the  southern  area  is  in  general  drier  and 
warmer,  with  somewhat  more  xerophytic  vegetation  than  the 
northern  area,  and  that  differences  in  vegetation  due  to  this  cause 
must  be  distinguished  as  well  as  possible  from  those  due  to  different 
distribution  of  rainfall. 


182 


BOTANICAL  GAZETTE 


[SEPTEMBER 


Evaporating  power  of  the  air  has  not  been  subject  to  geographic- 
statistical  treatment,  since  there  are  no  data.  It  was  beyond  the 
scope  of  the  present  study  to  have  attempted  instrumental  investi- 
gation on  a scale  large  enough  to  be  of  value.  There  seems  to  be 
little  doubt  that,  as  a geographic  factor  in  regions  of  continental 
climate,  evaporating  power  of  the  air  is  of  about  the  same  indicator 
value  as  rainfall.  It  varies  geographically  about  as  does  rainfall, 
in  inverse  ratio,  since  evaporating  power  is,  in  large  measure,  a 
function  of  rainfall.  This  same  inverse  ratio  seems  to  hold  in 
seasonal  distribution  as  well  as  geographically.  This  may  be  seen 
from  the  graphs  of  Weaver  (20),  and  from  data  obtained  by 
Cooper  in  a study  of  chaparral  in  California. 

As  a local  factor  evaporation  is  separately  treated  in  the  dis- 
cussion of  local  distribution  of  vegetation. 

For  further  discussion  of  the  climatology  of  Colorado  in  rela- 
tion to  vegetation  the  reader  is  referred  to  the  articles  of  Shantz 
(15,  16),  Ramaley  (12),  and  Robbins  (13,  14).  Data  may  be 
had  from  the  bulletins  of  the  United  States  Weather  Bureau, 
Colorado  College,  the  Agricultural  Experiment  Station  at  Fort 
Collins,  and  the  Bureau  of  Plant  Industry. 

Local  distribution  of  vegetation 

Physical  factors. — Local  physical  conditions  affecting  plant 
distribution  are  those  concerned  with  substratum  and  soil;  with 
topography , especially  local  position  with  respect  to  surroundings, 
and  slope  of  surface,  as  regards  both  steepness  and  direction  of 
exposure;  and  with  local  variation  in  atmospheric  conditions , as 
controlled  primarily  by  topography.  The  variability  of  these 
factors  within  the  region  is  great,  and  their  interactions  are  com- 
plex. Descriptions  of  the  soil,  topography,  atmospheric,  condi- 
tions, etc.,  of  the  different  parts  of  the  region  are  scattered  through 
both  systematic  and  regional  sections  of  this  study,  and  a lengthy 
discussion  at  this  point  would  be  out  of  place.  A few  references 
to  other  parts,  and  certain  incidental  comments,  may  here  be  made. 

The  character  of  the  substratum,  and  some  of  its  influences  in 
determining  soil  conditions  and  topography,  are  indicated  in  the 


1919]  VESTAL— PHYTOGEOGRAPHY  OF  COLORADO  183 

account  of  the  sedimentary  area.  The  contrast  between  the  granite 
soil  of  the  foothills  and  the  soil  of  sedimentary  origin  lying  just  out- 
side, with  its  slight  selective  action  on  flora  and  vegetation,  has 
also  been  noted.  Other  mentions  of  soils,  especially  as  regards  soil 
texture,  are  scattered. 

Topography  is  systematically  treated  for  particular  regions 
and  smaller  areas  by  dividing  each  type  of  topographic  complex 
into  topographic  areas  or  habitats;  with  each  type  is  correlated  the 
particular  plant  community  or  the  several  communities  which 
accompany  it.  In  the  regional  section  will  be  found  similar  analyses 
of  the  cuesta,  high  mesa,  mesa- terrace,  and  flood-plain  complexes. 
Particular  physical  factors  controlled  by  local  position  and  by 
slope  are  mentioned  in  a former  article  (18). 

Atmospheric  factors  vary  locally  in  this  region  to  a probably  not 
very  great  extent,  but  even  slight  differences  may  be  critical, 
as  has  been  found  by  Cooper  in  the  California  chaparral.  The 
factor  of  greatest  influence  upon  plant  life,  and  the  one  most  readily 
measured,  is  the  evaporating  power  of  the  air,  the  value  of'  which 
represents  the  resultant  of  several  contributing  factors.  Local 
distribution  of  evaporating  power  is  believed  to  be  controlled 
primarily  by  differences  in  topography,  and  secondarily  by  differ- 
ences in  vegetation-cover.  That  is  to  say,  flatness  of  the  land 
surface  makes  for  comparative  uniformity  of  exposure  to  wind 
and  sun;  hilliness  causes  diversity  of  exposure.  Local  water  or 
wet-soil  surfaces  may  lower  evaporating  power  by  contributing 
much  water  vapor  to  the  air.  Topography  thus  determines  the 
original  local  distribution  of  evaporating  power.  This  original 
local  distribution  is  modified  by  vegetation-cover.  In  flat  country 
the  uniformity  is  changed.  Low  and  open  vegetation  lowers  evap- 
orating power  at  the  ground  surface  only  slightly,  but  mesophytic 
closed  forest  lowers  it  very  greatly  (Gates  3,4).  In  hilly  country  in 
not  too  humid  climates  the  originally  protected  ravines  and  shaded 
or  wind-sheltered  slopes  may  develop  mesophytic  vegetation  which 
still  further  lowers  evaporation,  while  the  originally  exposed  slopes 
and  summits  usually  remain  xerophytic.  Thus,  in  the  mountain- 
front  region  here  considered,  primary  environmental  differences 


184 


BOTANICAL  GAZETTE 


[SEPTEMBER 


due  to  topography  may  rather  thoroughly  control  vegetation  dis- 
tribution. In  such  cases  the  reaction  of  vegetation-cover  upon 
local  evaporation  conditions  may  merely  heighten  the  original 
topographically  determined  contrast  between  protected  and  ex- 
posed habitats.  Topography  governs  local  vegetation  distribution 
through  the  mediative  influence  of  a number  of  physical  factors, 
of  which  evaporating  power  is  one.  Depending  as  it  does  upon 
several  other  factors,  evaporation  forms  a convenient  index  of 
habitat,  but  is  not  in  itself  the  basic  controlling  condition.  For 
these  reasons  the  writer  has  subordinated  the  influence  of  evaporat- 
ing power  upon  local  distribution  to  that  of  topography. 

The  sudden  change  of  elevation  at  the  mountain-front  is  a 
topographic  condition  affecting  evaporating  power.  At  many 
places  the  hogbacks,  mesas,  and  outer  slopes  receive  no  direct 
sunlight  during  several  hours  before  sunset,  being  shaded  by  the 
higher  slopes  immediately  to  the  west.  This  contributes  to 
the  comparative  mesophytism  of  certain  mountain-front  stations 
where  the  descent  from  foothills  to  plains  is  more  than  ordinarily 
abrupt. 

Direction  of  exposure  affects  local  atmospheric  conditions  and 
vegetation  in  many  easily  observed  ways.  Cloudiness  and  showers 
occur  on  summer  afternoons  much  more  frequently  than  in  'the 
mornings,  as  Ramaley  has  noted.  East-facing  slopes  are  thus 
likely  to  be  drier  than  west-facing  slopes  (the  latter  are  less  fre- 
quent east  of  the  range-crest).  As  would  be  expected,  the  differ- 
ence between  north-  and  south-facing  slopes  is  considerable,  the 
latter  being  more  exposed  to  sun  and  conditions  favoring  rapid 
evaporation,  and  with  sparser,  more  xerophytic  vegetation.  In 
open  parts  of  the  foothills  where  slopes  are  quite  gentle  the  north- 
facing slopes  are  not  sufficiently  sheltered  from  sun  and  wind  to 
differ  in  vegetation  from  the  south-facing  slopes  in  any  marked 
degree.  Steep  north  slopes,  or  both  sides  of  steep  and  narrow 
ravines  which  run  down  to  the  north,  however,  are  quite  meso- 
phytic.  In  different  parts  of  so  large  a territory  the  combina- 
tions of  contrasting  vegetation  of  north  and  south  slopes  would 
be  expected  to  vary,  and  a few  of  them  are  listed  herewith  by  way 
of  illustration. 


1919] 


VESTAL— PHYTOGEOGRAPHY  OF  COLORADO 


185 


TABLE  VI 

Effects  of  direction  of  slope  upon  local  distribution 


Locality 

Vegetation  of  south-facing 
slope 

Vegetation  of  north-facing  slope 

Poudre  foothills 

Grassland 

Scattered  rock  pine,  with  more 
mesophytic  vegetation  infre- 
quent 

Foothills  near  Boulder.  . 

Grassland,  rock  pine, 
mixed  shrub 

Pseudotsuga,  canyon  forest,  rock 
pine,  mesophytic  grassland 
Cercocarpus,  with  very  scattered 
rock  pines  in  rocky  places; 
grassland  in  fine  soil  • 

Poudre  mountain-front . . 

Grassland 

Mountain-front  near 

Boulder 

Grassland,  mostly 

Grassland  with  rock  pine,  mixed 
shrub,  etc. 

South  of  Golden,  moun- 

tain-front   

Grassland  and  Cerco- 
carpus 

Grassland  with  scattered  rock 
pine  and  mixed  shrub 

Perry  Park 

Oak  and  grassland 

Rock  pine  and  Pseudotsuga 

Palmer  Lake 

Oak 

Pseudotsuga 

Southern  mountain-front 

in  general 

Pinyon-cedar,  dry  grass- 
land, and  scattered 
oaks 

Closer  and  taller  oak  growth  with 
rock  pines 

Factors  other  than  physical  conditions  oe  habitat. — 
If  the  physical  conditions  which  determine  the  habitat  and  all 
their  interactions  and  variations  were  fully  known,  however,  the 
local  distribution  of  plant  communities  as  observed  would  only 
partially  be  explained.  Within  even  a very  small  part  of  the 
region  studied  correlations  between  physical  habitats  and 
vegetation-types  must  not  be  too  closely  drawn.  The  rock  pine, 
for  example,  grows  in  any  soil  or  on  any  slope;  its  presence  or 
absence  in  any  particular  situation  is  not  alone  a matter  of  physical 
conditions  there  and  then  operative.  Local  distribution  of 
vegetation-types  in  these  partly  unstable  and  locally  very  diverse 
situations  depends  also  on  at  least  three  other  sets  of  conditions: 
(1)  range  of  toleration,  in  individual  species  or  groups  of  species, 
of  variation  of  physical  conditions;  (2)  local  historic  factors,  physi- 
cal and  vegetational,  which  have  been  operative  in  any  given  spot 
(these  often  cannot  be  determined);  (3)  accident  of  seed 
distribution  and  germination.  For  these  reasons  it  seems  best  to 
characterize  the  vegetation-units,  in  most  cases,  from  the  vegeta- 
tion itself  rather  than  from  habitat.  There  can  be  no  question 


i86 


BOTANICAL  GAZETTE 


[SEPTEMBER 


that,  in  general,  local  variation  of  present  physical  conditions  of 
the  habitat  governs  to  a considerable  degree  the  distribution  of 
plant  communities,  but  the  need  of  at  least  recognizing  these  other 
sets  of  factors  should  be  emphasized.  It  must  be  further  seen 
that,  in  the  invasion  of  a new  habitat,  representatives  from  more 
than  one  plant  community  can  be  successful  in  establishing 
themselves,  resulting  in  mixed  vegetation-types.  In  fact,  probably 
the  greater  part  of  the  area  studied  is  occupied  by  mixed  associations 
or  mictia  (Clements).  Even  areas  of  established  vegetation  are 
usually  open  enough  to  permit  the  continual  ecesis  within  them  of 
new  plant  immigrants  from  quite  different  communities.  This 
diversity  is  likely  to  be  relatively  enduring,  for  plant  competition 
usually  does  not  here  operate  to  exclude  all  but  a single  type  of 
dominants.  The  opposite  relation  between  plants,  which  may  be 
called  accommodation,  is  as  greatly  in  evidence.  The  control 
exerted  by  vegetation  upon  the  physical  environment  is  slight  over 
the  generally  xerophytic  mountain-front  region. 

A second  factor  contributing  to  the  mixed  effect  is  the  frequent 
extremely  local  variability  of  physical  conditions  within  the  habitat. 
This  might  be  called  mosaic  variability,  and  its  effect  a mosaic 
mixture  of  vegetation-types.  The  influence  of  large  surface  rocks 
partly  imbedded  in  fine  soil,  allowing  the  growth  of  comparatively 
mesophytic  plants  in  a rather  constant  interspersal  with  xerophytes 
over  a considerable  area,  may  be  cited  as  an  example. 

Vegetation- types  and  their  distribution 

Since  the  plant  communities  have  been  described  separately 
in  the  two  articles  preceding  this,  their  systematic  characterization 
here  may  be  condensed  very  considerably.  A tabular  view  of  the 
communities,  giving  some  idea  of  their  general  character  and  of 
their  distribution  in  the  main  geographic  divisions  of  the  region 
studied,  is  shown  in  table  VII. 

Some  of  the  more  important  features  of  the  particular  associa- 
tions may  now  be  noted.  Details  and  references  to  other  accounts 
may  be  found  in  the  articles  preceding.  The  general  appearance 
of  certain  vegetation- types  may  be  seen  in  fig.  17. 


1919]  VESTAL— PHYTOGEOGRAPHY  OF  COLORADO  187 

Lichen  association. — Lichens,  especially  Rinodina,  Lecanora, 
and  Parmelia  conspersa , partly  cover  the  dry  rock  surfaces,  espe- 
cially granites  in  the  foothills  and  the  craggy  outcrops  and  loose 
surface  rocks  of  the  mountain-front.  Rock  exposures  are  infre- 
quent in  the  plains  proper. 

TABLE  VII 

Conspectus  of  associations 


Foothills 

Mountain-front 

Plains 

Thallus  vegetation 

Lichen  association 

Lichen  association 

(Lichen  association) 

Grassland 

Extensive,  climatic 

Local,  edaphic 

Foothills  grassland 

Mixed  short-grass 

[Wheat-grass 

\Stipa-Aristida 

Bunch-grass 

Prairie-grass 

Short-grass 

[Wheat-grass 
\Stipa-A  ristida 

Bunch-grass 

(Local,  infrequent, 
prairie-grass-like 
communities) 

(meadow  type) 

(mixed  type) 

Primitive  grassland 

A rtemisia-Gutierrezia 
consocies 

Primitive  bunch-grass 

Less  xerophytic 

Bunch-grass,  plus  a 
foothills  element 
Mesophytic  grassland 

Primitive 

forest  herb  type 
meadow  type 

[Foothills  primitive 

1 grassland 

\ Artemisia  frigida  con- 
[ socies 

meadow  type 
mixed  type 

Primitive  grassland 

Artemisia  frigida  con- 
socies 

Primitive  bunch-grass 

Shrub  vegetation 

Xerophytic 

Cercocarpus  association 

[Mixed  shrub  association 

\Arctostaphylos 

Ceanothus  association 
[ Symphoricarpos 

[ Chrysothamnus-  Sarcoba- 
\ tus  association 

Chrysothamnus-  Sarcoba- 
tus  association 

Xerophytic  to  mesophytic . . 

[Cercocarpus  association 
Mixed  shrub  association 

Symphoricarpos 

(Local  shrub  communi- 
ties) 

(, Symphoricarpos ) 

Tree  vegetation 
Coniferous 

Xerophytic 

Pinyon-cedar  associa- 
tion 

Rock  pine  association 
Pseudotsuga  association 

[Oak  association 
| Populus-Salix  associa- 
1 tion 

[Canyon  forest 
\ (Aspen  association) 

Pinyon-cedar  associa- 
tion 

Rock  pine  association 
( Pseudotsuga  associa- 
tion) 

Oak  association 
Populus-Salix  associa- 
tion 

Canyon  forest 

Less  xerophytic 

Relatively  mesophytic. . . 
Deciduous 

Xerophytic  to  meso- 
phytic   

Populus-Salix  associa- 
tion 

Relatively  mesophytic . . . 

Associations  with  equivalent  or  similar  representation  in  plains,  mountain-front,  and  foothills  areas 
are  shown  on  the  same  horizontal  line.  Very  local  or  poorly  developed  representation  of  a community  in 
a particular  zone  is  indicated  by  parentheses. 


Mixed  grasslands  and  short-grass. — The  shallow-rooted  short- 
grasses,  Bouteloua  and  Bulbilis,  dominate  the  compacted  fine  soil 
surface  of  most  of  the  plains,  as  the  well  known  short-grass  associa- 
tion. Bouteloua  alone,  with  admixture  of  plants  of  different  physio- 
logical and  geographic  character,  is  the  important  element  of  dry 


i88 


BOTANICAL  GAZETTE 


[SEPTEMBER 


VESTAL— PHYTOGEOGRAPHY  OF  COLORADO 


1919] 


189 


grassland  in  the  debris-covered  soil  of  the  mesas  and  outwash-plains 
of  the  mountain-front  (short-grass  mixed  association).  The  foot- 
hills mixed  grassland,  very  similar  to  the  mixed  short-grass,  is 
typical  over  the  coarse  surface  of  the  granitic  foothills. 

Wheat-grass. — This  taller  but  also  shallow-rooted  grass,  Agro- 
pyron  Smithii,  dominates  areas  of  loose  clay  in  the  mountain-front 
and  plains.  Its  ecological  character  is  not  well  understood. 

Stipa-Aristida  association. — These  tufted  xerophytic  grasses  of 
coarse  soil  occur  frequently  but  not  extensively,  together  or  singly, 
with  other  rather  deep-rooted  plains  xerophytes. 

Bunch-grass. — Tufted  perennial  and  deep-rooted  grasses, 
depending  on  continued  moisture,  such  as  Andropogon  scoparius, 
A.  furcatus,  Sorghastrum  nutans , etc.,  are  notable  through  most 
of  the  prairie  region,  almost  absent  in  dry  plains,  but  abundant  in 
the  rain  belt  of  eastern  Colorado;  at  the  mountain-front  and  in 
the  foothills,  scatteringly  in  the  north,  but  very  frequent  at  the 
Platte-Arkansas  divide  and  southward  into  New  Mexico.  In 
the  foothills  there  are  grasses  of  similar  habit  which  mingle  with 
the  prairie  bunch-grasses. 

Mesophytic  grasslands. — Mesophytic  herbaceous  growths  are 
made  up  partly  of  prairie  plants  and  partly  of  Rocky  Mountain 
forest  plants.  The  latter  element  is  very  considerable  in  occasional 
foothill  ravines.  Meadow  growths  of  both  foothills  and  mountain- 
front,  in  moist  soil,  with  showy  flowering  plants  like  Delphinium , 
Cerastium,  Castilleja,  Orthocarpus,  etc.,  are  conspicuous  in  early 
summer,  but  not  very  frequent.  The  mountain-front  in  many 
places  shows  a mixed  grassland  much  like  that  of  eastern  prairie, 
which  has  been  called  western  prairie-grass.  It  has  plants  of  the 
mixed  short-grass,  with  components  from  bunch-grass  and  eastern 
prairie  or  forest  border,  with  some  foothills  mesophytes,  and  a few 
plants  characteristic  of  the  mountain-front,  like  Stipa  viridula. 

Primitive  grasslands. — Early  stages  of  grassland  developing 
in  areas  recently  bared,  or  remaining  for  long  in  loose  shifting 
slopes,  are  frequently  seen.  Prostrate  plants  with  heavy  taproots 
(rosette  plants)  are  common.  Gravel-slides  in  the  foothills  and 
dry  stony  crests  of  mesas,  buttes,  and  ridges  in  the  plains  and 
mountain-front  are  the  typical  habitats.  The  Bouteloua  hirsuta 


BOTANICAL  GAZETTE 


[SEPTEMBER 


190 

and  the  Artemisia  frigida  consocies  may  be  mentioned  specially. 
The  last  is  closely  allied  to  the  Gutierrezia- Artemisia  association  of 
the  plains,  very  widespread,  and  continuing,  at  the  expense  of 
short-grass,  with  heavy  grazing.  In  mountain-front  and  plains 
the  primitive  bunch-grass  association,  with  Panicum  virgatum , 
Sporobolus  cryptandrus,  Stipa  Vaseyi,  Eriocoma,  etc.,  occupies  sandy 
or  loose-soil  habitats  recently  disturbed. 

Chrysothamnus-Sarcobatus  association. — The  shrubby  composite, 
Chrysothamnus  (rabbit-brush),  and  the  chenopodiaceous  grease- 
wood  occupy  loose  soil,  mostly  alkaline  areas,  on  certain  slopes 
in  the  mountain-front,  and  are  particularly  abundant  in  stream- 
bottoms  in  the  southern  plains. 

Cercocarpus  association. — Mountain-mahogany,  of  the  rose 
family,  is  the  only  dominant  in  the  open  shrub  growth  of  the 
mountain-front  and  outer  foothills,  in  very  dry  exposed  situations 
and  usually  stony  soil.  In  the  interstices  between  shrubs  are 
plants  of  primitive  grassland  or  mixed  short-grass. 

Mixed  shrub  association. — -This  is  a heterogeneous -assemblage  of 
shrubs,  ranging  from  xerophytic,  like  Rhus  trilobata,  to  relatively 
mesophytic  forms,  like  Crataegus  coloradensis,  in  sheltered  situa- 
tions. The  same  species  range  through  a variety  of  habitat  condi- 
tions, and  may  form  a community  either  as  shrubs  or  trees.  The 
mixed  shrub  grades  into  the  canyon  forest. 

Arctostaphylos  association. — The  well  known  and  widespread 
bearberry  forms  its  characteristic  mats  in  the  foothills,  mostly  on 
compacted  gravelly  floors.  It  is  more  abundant  in  the  upper 
foothills,  in  open  places  among  the  scattered  pines.  Its  congener, 
Juniperus  communis  sibirica,  is  present  but  infrequent. 

Ceanothus  association. — Ceanothus  Fendleri  forms  low  matlike 
ground-cover  in  the  lower  foothills,  similar  to  that  of  Arctostaphylos , 
though  it  is  not  evergreen,  is  of  more  southerly  distribution,  and 
ranges  into  drier  and  more  exposed  habitats.  It  favors  the  estab- 
lishment of  seedling  mesophytes,  and  plays  a part  in  revegetation 
of  burned  areas. 

Symphoricarpos  association. — The  buckbrush,  as  it  is  called, 
occupies  moist  fine  soil,  and  invades  grassland  in  the  mountain- 
front  and  foothills,  as  well  as  in  the  eastern  prairie,  in  favorable 


1919]  VESTAL— PHYTOGEOGRAPHY  OF  COLORADO  19 1 

situations  such  as  draws  and  seepage  areas  of  slopes.  It  in  turn  is 
frequently  displaced  by  taller  woody  vegetation. 

Rock- pine  association .- — Pinus  scopulorum  is  the  important  tree 
# of  the  foothills.  It  ranges  into  very  variable  habitats,  and  is 

structurally  variable  in  accordance.  It  forms  infrequent  close 
stands,  but  in  most  places  is  scattered,  the  ground  between  the 
9 trees  being  occupied  by  foothills  mixed  grassland,  Ceanothus, 

Arctostaphylos,  etc.  It  is  frequent  in  rocky  crests,  etc.,  in  the 
mountain-front,  except  in  the  south,  where  it  is  commonly  replaced 
by  pinyon.  It  extends  very  locally  into  the  plains  in  broken 
country,  on  butte-crests,  etc.,  and  on  the  elevated  Platte-Arkansas 
divide. 

Piny  on-cedar  association. — Pinus  edulis  and  Juniperus  mono- 
sperma  are  important  xerophytic  conifers  of  the  southern  mountain- 
front  and  lower  foothills  north  to  the  Garden  of  the  Gods,  and 
extending  into  the  southern  plains  on  mesa-crests,  canyon-walls, 
and  bluffs  of  broad  valleys.  The  soil  is  usually  rocky  or  gravelly. 
< The  trees  are  low  and  rounded,  and  do  not  form  a closed  assemblage. 

Pseudotsuga  association. — Pseudotsuga  mucronata  forms  the 
mesophytic  or  relatively  mesophytic  coniferous  forest  of  the  region, 
and  is  confined  to  sheltered  ravines  and  steep  north  slopes  in  the 
foothills.  It  is  infrequent  at  the  mountain-front. 

Oak  association. — Small  trees  of  the  white-oak  group,  of  uncer- 
tain taxonomic  affinity,  form  dense  copses  or  open  woods  in  the 
lower  foothills  and  in  the  mountain-front  about  as  far  north  as 
Platte  Canyon.  In  places  grazing  destroys  the  oak  slowly  and 
allows  increase  of  grassland.  The  undergrowth  of  mesophytic 
oak  areas  is  much  like  that  of  the  canyon  forest. 

Populus-Salix  association. — In  stream-side  areas  of  the  foothills 
Populus  angustifolia  and  4 or  5 common  willow  species  are  frequent. 
Outside  the  mountains  Populus  Sargentii,  and  in  the  south 
Populus  Wislizeni , replace  the  narrow-leaf  cottonwood.  Cotton- 
woods extend  eastward  info  the  plains  for  many  miles  along 
watercourses. 

> Canyon  forest: — The  deciduous  trees  of  the  foothill  canyons  and 

of  ravines,  etc.,  in  the  mountain-front,  include  Alnus  tenuifolia , 
Betula  fontinalis  (these  two  common  along  mountain  streams), 


192 


BOTANICAL  GAZETTE 


[SEPTEMBER 


Amelanchier  alnifolia,  Prunus  pennsylvanica,  P.  americana , P. 
demissa,  Crataegus  color adensis  et  spp.,  Robinia  neomexicana  (in 
the  south  only),  Acer  glabrum,  and  A.  Negundo,  with  a few  others. 
A few  shrubs  are  present  and  a variable  undergrowth,  with  one 
typical  aspect  of  Viola  canadensis  Rybd.,  Hydrophyllum,  and 
Galium.  A Ligusticum  is  very  abundant  in  places. 

Aspen  association. — Populus  tremuloides  is  restricted,  in  all  but 
the  highest  parts  of  the  foothills,  to  relatively  mesophytic  ravines. 
It  does  not  come  up  abundantly  following  burning  of  the  pine 
forest,  as  is  true  in  the  higher  elevations  and  farther  north. 

Eastern  Illinois  State  Normal  School 
Charleston,  III. 

LITERATURE  CITED 

1.  Davis,  W.  M.,  The  Colorado  Front  Range.  Ann.  Ass.  Amer.  Geog. 
1:21-83.  1911. 

2.  Fenneman,  N.  M.,  Geology  of  the  Boulder  district,  Colorado.  U.S. 
Geol.  Surv.  Bull.  265.  pp.  101.  1905. 

3.  Gates,  F.  C.,  The  relation  between  evaporation  and  plant  succession  in  a 
given  area.  Amer.  Jour.  Bot.  4:161-178.  1917. 

4.  Gleason,  H.  A.,  and  Gates,  F.  C.,  A comparison  of  rates  of  evaporation 
in  certain  associations  in  central  Illinois.  Bot.  Gaz.  53:478-491.  1912. 

5.  Johnson,  W.  D.,  The  High  Plains  and  their  utilization.  Ann.  Repts. 
U.S.  Geol.  Surv.  21,  part  4,  pp.  599-741;  22,  part  4,  pp.  631-669.  1900, 
1901. 

6.  Lee,  W.  T.,  The  origin  of  the  debris-covered  mesas  of  Boulder,  Colorado. 
Jour.  Geol.  8:504-511.  1900. 

7.  Marvine,  A.  R.  [The  sedimentary  rocks  east  of  the  Front  Range  (chap, 
ii,  pp.  93-137,  in  Marvine’s  report)],  in  Hayden,  F.  V.,  Ann.  Rept.  U.S. 
Geol.  and  Geog.  Surv.  Terr,  for  1873,  embracing  Colorado.  718  pp. 
Washington,  1874. 

8.  Ramaley,  F.,  Plant  zones  in  the  Rocky  Mountains  of  Colorado.  Science 
26:642-643.  1907. 

9.  , Botany  of  northeastern  Larimer  County,  Colorado.  Univ.  Colo. 

Studies  5: 119-131.  1908. 

10.  , Dry  grassland  of  a high  mountain  park  in  northern  Colorado. 

Plant  World  19: 249-270.  1916. 

11.  , Vascular  plants  of  the  Tolland  region  in  Colorado.  Univ.  Colo. 

Studies  12:27-51.  1917. 

12.  Ramaley,  F.,  Robbins,  W.  W.,  and  Dodds,  G.  S.,  Studies  in  mesa  and  foot- 
hill vegetation,  I.  Univ.  Colo.  Studies  6:11-49.  1908. 


VESTAL— PHYTOGEOGRAPHY  OF  COLORADO 


193 


1919] 

13.  Robbins,  W.  W.,  Climatology  and  vegetation  in  Colorado.  Bot.  Gaz. 
49:256-280.  1910. 

14.  , Native  vegetation  and  climate  of  Colorado  in  their  relation  to 

agriculture.  Bull.  224,  Agric.  Exp.  Sta.  of  the  Colo.  Agric.  Coll.  56  pp. 
1917. 

15.  Shantz,  H.  L.,  A study  of  the  mesa  region  east  of  Pike’s  Peak:  the  Boute- 
loua  formation.  Bot.  Gaz.  42:16-47,  179-207.  1906. 

16.  , Natural  vegetation  as  an  indicator  of  the  capabilities  of  land  for 

crop  production  in  the  Great  Plains  area.  U.S.  Dept.  Agric.,  Bur.  PL 
Industry,  Bull.  201.  pp.  100.  1911. 

17.  Vestal,  A.  G.,  Prairie  vegetation  of  a mountain-front  area  in  Colorado. 
Bot.  Gaz.  58:377-400.  1914. 

18.  , Foothills  vegetation  in  the  Colorado  Front  Range.  Bot.  Gaz. 

64:353-385-  1917. 

19.  Ward,  R.  De  C.,  Rainfall  types  of  the  United  States.  Geog.  Review 
4:131-144.  1917. 

20.  Weaver,  J.  E.,  Evaporation  and  plant  succession  in  southeastern  Wash- 
ington and  adjacent  Idaho.  Plant  World  17:273-294.  1914. 


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